The use of glazes to decorate articles of glass and ceramic materials goes back into antiquity. Glazes are, essentially, glassy or vitreous coatings which not only can improve the aesthetic appearance of the articles to which such as applied, but also, in the case of ceramic articles, provide a non-porous, impermeable surface coating. In general, glazes have been defined as clear glasses; colored glazes have been defined as clear glasses with colorants dissolved therein; and enamels have been defined as glazes containing pigments or particles of other materials suspended therein.
Glazes and enamels are customarily applied to the surface of a glass, glass-ceramic, or ceramic article in the form of very finely-divided particles or powder, commonly known in the art as "frit," suspended in a volatile liquid vehicle to make a paste or slurry. This paste or slurry is then first fired at a relatively low temperature to burn off the vehicle (normally an organic liquid), and thereafter at a higher temperature to fuse the frit into a strongly-adherent, continuous film on the surface of the article.
Frits operable as glazes and enamels normally exhibit five criteria:
First, the firing or maturing temperature of the frit, i.e., the temperature at which the frit will flow sufficiently to yield a smooth uniform coating on an article, will generally be low enough to avoid thermal deformation of the article;
Second, the linear coefficient of thermal expansion of the frit will be compatible with that of the article being coated to prevent crazing and/or spalling; advantageously, the coefficient of thermal expansion of the frit will be a little below that exhibited by the article such that the fired coating will be under compression when the article is cooled to room temperature;
Third, the frit will commonly demonstrate good resistance to weathering and to attack by acids and alkalies; corrosion of the coating can lead to loss of gloss, the generation of haze and/or iridescence, the development of porosity, and/or other effects deleterious to the appearance or physical character of the coating;
Fourth, the frit will customarily display good glass stability, i.e., the frit will resist devitrification during firing, unless it is designed to function as a thermally devitrifiable frit; and
Fifth, where a glossy decorative glaze or enamel is sought, the refractive index of the frit must be high.
Most commercially-marketed glazes and enamels contain substantial concentrations of lead oxide (PbO) and, in some instances, cadmium oxide (CdO) or a combination of PbO and CdO. Those oxides have the capability of softening the frit, i.e., lowering the melting point and, hence, the temperature for fusing PG,4 the frit onto an article, and, in addition, of raising the refractive index of the frit. CdO has also been employed as a colorant in certain frits. Both of those ingredients, however, are subject to the disadvantage of being toxic. hence, care must be exercised by workers using those materials and glazes and enamels which contain those metals must exhibit extremely good resistance to attack by acids and bases when used in contact with food. Thus, release of any significant amount of those metals must be avoided.
In view of the toxicity inherent in lead and cadmium, considerable research has been undertaken to discover glazes and enamels free of those metals, but which still satisfy the above five criteria. The following patents are illustrative of such research.
U.S. Pat. No. 4,084,976 describes lead-free glazes especially designed for use with alumina bodies. Those glazes consisted essentially, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 50-54 BaO 2-5 Al.sub.2 O.sub.3 5-8 SrO 5-8 B.sub.2 O.sub.3 6-12 ZnO 1-2 CaO 4-6 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 4-6 MgO 2-8 ______________________________________
U.S. Pat. No. 4,120,733 discloses lead-free glazes statedly an improvement upon the compositions of U.S. Pat. No. 4,084,976 and consisting essentially, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 48-54 CaO 2-3 Al.sub.2 O.sub.3 7-11 ZnO 2-2.5 B.sub.2 O.sub.3 16.5-20 Na.sub.2 O 4.25-5.25 CaO 4-6 K.sub.2 O 0.4-1 BaO 11-14 ______________________________________
U.S. Pat. No. 4,224,074 presents lead- and cadmium-free glazes consisting essentially, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 29-55 Na.sub.2 O 4-20 Al.sub.2 O.sub.3 2-8 Li.sub.2 O 0-7 B.sub.2 O.sub.3 7-31 Na.sub.2 O + Li.sub.2 O 6-24 ZrO.sub.2 5-16 F 0.75-4 ______________________________________
U.S. Pat. No. 4,282,035 concerns lead- and cadmium-free glazes consisting essentially, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 51-60 Na.sub.2 O 0-5 B.sub.2 O.sub.3 4.5-8 K.sub.2 O 0-5 BaO 0-13 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 1-5 SrO 0-18 MgO 0-6 BaO + SrO 6-30 CaO 0-12 ZrO.sub.2 4-8 Bi.sub.2 O.sub.3 0-10 Al.sub.2 O.sub.3 5-8 MgO + CaO + Bi.sub.2 O.sub.3 0-20 Li.sub.2 O 0-4 ______________________________________
U.S. Pat. No. 4,285,731 defines lead- and cadmium-free glazes consisting essentially, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 35-47 SrO 0-8 B.sub.2 O.sub.3 5.5-9 MgO 0-5 BaO 24-42 CaO 0-4 TiO.sub.2 1.5-4 ZnO 0-10 ZrO.sub.2 6-10 Bi.sub.2 O.sub.3 0-8 Li.sub.2 O 1-5 SrO + MgO + CaO + ZnO + Bi.sub.2 O.sub.3 0-10 ______________________________________
U.S. Pat. No. 4,311,504 describes lead-free glazes having the same base compositions as the glazes defined in U.S. Pat. No. 4,285,731, but also containing 2-10% cadmium sulfoselenide to impart a bright red coloration to the fired glaze.